Repeaters, distributed antenna systems, and similar systems are communication systems that are used to extend wireless signal coverage into areas where the RF signal penetration from base transceiver stations (BTS) is often limited or absent. For example, an overall wireless communication system may consist of a plurality of base transceiver stations (BTS) or base stations that communicate with each other and with user equipment, such as cellular phones, to provide a defined coverage area. In such coverage areas, there are often smaller geographical areas that have very low signal coverage, as provided by one or more of the base stations. Those areas, for example, might be inside buildings, in tunnels, in shadowed areas that are behind mountains or other obstructions, and in underground train systems, as well as various other isolated areas. Rather than simply implementing another costly and large base station to provide coverage in such low signal areas, repeaters and distributed antenna systems are often utilized.
One particular issue to be addressed within a repeater is the feedback signal that comes directly from the coverage antenna back to the donor antenna (or vice-versa for the uplink traffic). Because the feedback signal in an on-frequency repeater system is generally the same frequency as the input signal, it is considered co-channel interference, and cannot be removed by conventional filtering techniques. The feedback path can occur via a direct path, a path directly from the transmit antenna to the receive antenna, or a reflected path, a path where the signal is reflected off one or more object before being received. The direct path and the reflected paths together are called multipath coupling interference, or more simply multipath interference. Under specific conditions, the components of the multipath signals are amplified upon retransmission, overwhelming the desired signal or leading to system oscillation or other system instability.
To remove the multipath signals without corrupting the desired uplink or downlink signals, the multipath signals must be subtracted from any received signal or input signals so as to only repeat the desired signals.
It would therefore be desirable for the communication system or a portion thereof to mitigate such multipath interference to prevent the retransmission thereof.